The imager used in this work consists of a Compton camera that utilizes three PSDs distributed in two parallel detection planes, Scatter (S) and Absorber (A), as shown in Fig. REF . Each PSD contains a LaCl\(_{3}\) (Ce) monolithic crystal with a square-cuboid shape and a base surface of 50\(\times \) 50 mm\(^2\) . The LaCl\(_{3}\) (Ce) is hygroscopic and thus it is encapsulated in an aluminum housing. The crystal base is coupled to a 2 mm thick quartz window, which is optically coupled to a silicon photomultiplier (SiPM) from SensL (ArayJ-60035-64P-PCB). The photosensor features 8\(\times \) 8 pixels over a surface of 50\(\times \) 50 mm\(^2\) . A 10 mm thick crystal is used for the PSD in the S-plane. Two 25 mm thick crystals are utilized for the two PSDs placed in a vertical stack for the A-plane (see Fig. REF ). In total, 192 SiPM channels are biased and readout by means of front-end and processing PETsys TOFPET2 ASIC electronics [1]}. In order to minimize gain shifts due to changes in the temperature of the experimental hall, every ASIC is thermally coupled to a refrigeration system composed by Peltier cells (see Ref. [2]} for further details).
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